Canon PIXMA iP5200 - error message "more than one cyan cartridgeinstalled"

I have a Canon PIXMA iP5200 that I'm generally pleased with. However,
recently it has been giving me an error message when I try to print
that I have more than one cyan cartridge installed. I have only one
and it is in the correct slot. It is not empty as far as I can tell -
the fiber part of the reservoir still appears saturated at about half-
height. If I remove and reinsert the cyan cartridge repeatedly and
sometimes if I power down the printer and back on, it seems to clear
the error and prints normally, but I'm never certain that it's going
to come out of it, or how much fiddling it will take. Besides, it
should just work!

I have DLed and installed the latest driver (2006), but the problem
continues. The system is a Dell Dimension 4600 running Windows XP Home
Edition, Ver. 2002, SP3.

Any suggestions regarding the source of the problem and the solution,
or even a pointer to another possible site would be very appreciated.

Thanks!
Optiker